Polycyclic aromatic hydrocarbons (PAH) are a class of environmental chemicals that are known to cause toxicity and cancers in humans. The molecular changes induced by exposure to PAH are mediated by the aryl hydrocarbon receptor (AHR). In particular, the AHR mediates up-regulation of phase I and phase II xenobiotic metabolizing enzymes that can cause both bioactivation and detoxification of PAH. Indeed, there is a fine balance between bioactivation and detoxification of chemical carcinogens that are directly influenced by AHR-dependent signaling. Emerging evidence indicates that PPAR?/? may modulate AHR activity in skin/keratinocytes, and that this interaction will significantly alter the balance between bioactivation and detoxification leading to differences in the susceptibility to chemically-induced toxicity. For Specific Aim 1, we will determine the mechanisms by which PPAR?/? can modulate AHR activity with an emphasis on receptor heterodimer translocation and critical transcriptional events necessary for increasing expression of xenobiotic metabolism that are necessary for bioactivation and detoxification of PAH. For Specific Aim 2, we will determine if PPAR?/? can functionally alter PAH-induced skin carcinogenesis mediated by AHR. Results from these studies will elucidate novel regulatory mechanisms of AHR-dependent signaling that are central in mediating the toxic and carcinogenic effects caused by exposure to environmental PAH. Importantly, results from these studies might help explain the significant variation in AHR-dependent function in humans, since no functional polymorphisms in the Ahr have been identified.